It is my personal belief that something has plagued some of the 06 trucks causing slow cranking. When I say slow, i mean slower than every other earlier or later year model truck. I have slowly played with and diagnosed mine for years.
I've been through two sets of batteries, checked voltage drop across ALL my cables during cranking, plugged the truck in or left it cold... It doesn't matter. My truck has always spun over slower than other trucks.
The only time I've ever noticed it spin slightly faster (similar but still not quite as fast as others) was after replacing the solenoid contacts. I immediately noticed a change after this, but it seemed to only last for a month or so. That was a couple years ago, and ever since it still spins over as slow as it always did. I also had a starter shop go through it, who found nothing wrong.
I'm not saying you don't have an underlying problem of your own, but I've been battling mine for 5+ years, mainly because I attribute this slow cranking speed to my intermittent 'hiccup' start, that also tends to happen most often around 55-60*F. My next plan is to swap starters with a friend with an 08 that sounds normal and see what happens.